I think it's already a given that they know how to use the Rider Kick. :laugh::laugh::laugh:
The only Rider who had to be taught his/her first Rider Kick from what I've seen is Fourze. For those Riders that started out the series knowing nothing, Gaim, and OOO already had a Rider Kick ready. Everyone else, I think, had been a Rider for some time before the start of the series.
There is no one teach Gentarou how to rider kick, he just playfully activate Rocket and Drill Module in midair and go with kick. Same thing happened to Kouta, he already in midair after kick his Pine Ball and just go with kick.
They just coincidentally in best position to kick.
OOO maybe, but he is given enough 2 eps information to use scanner on his medal, and it extend his leg, and he indeed already use flurry kick when get enhanced by Batta Medal.
Kuuga is the only heisei rider who practice his rider kick.
what happen to Shinnosuke is same with Ryuki
Doing that complex Rider kick without enough information